1. Field of the Invention
The present invention relates to a transmission power control circuit, and more particularly, to a transmission power control circuit that is used in a two-way radio communication device and that can maintain transmission power constant while performing transmission.
2. Description of the Background Art
Currently, as the Internet has vastly developed, two-way radio communication devices that take on the infrastructure of the Internet have become widely used. Major examples thereof are portable telephones, two-way satellite communication, and ground microwave communication. However, transmission power in such two-way radio communication devices must be strictly controlled in order to respond to a request from a receiving end, i.e. a base station or a satellite, and to reduce unwanted emission of spurious and noise. For that purpose, a transmission power control circuit maintaining the transmission power constant is generally integrated in a transmission portion of a two-way radio communication device.
FIGS. 8 and 9 are block diagrams showing a conventional transmission power control circuit. In FIG. 8, a high-frequency signal is input into a transmission-level adjusting portion 1, where the signal is adjusted in its level, and is further input into a distribution portion 2. Distribution portion 2 creates two signals from the input high-frequency signal for distribution, one of which is output as a transmission signal, and the other one of which is applied to a detection portion 3.
At detection portion 3, as shown in FIG. 9, the input high-frequency signal is detected and rectified by a diode 6, a capacitor 7 and a resistance 8, and a component of the signal is converted into a direct-current voltage (hereinafter referred to as DC voltage). The DC voltage is then applied to a comparator portion 4. Comparator portion 4 compares respective values of the DC voltage from detection portion 3 with an arbitrary DC voltage supplied from a reference voltage generating portion 5. If the DC voltage from detection portion 3 is lower than the DC voltage from reference voltage generating portion 5, comparator portion 4 controls the level of the high-frequency signal output from transmission-level adjusting portion 1 to be high, and to be low if otherwise.
Transmission-level adjusting portion 1 is realized by a circuit such as a variable gain amplifier and a variable attenuator. The high-frequency signal output from transmission-level adjusting portion 1 is distributed again by distribution portion 2 and applied to detection portion 3. By repeating this operation, the value of the DC voltage output from detection portion 3 eventually converges at the same value as that of the DC voltage from reference voltage generating portion 5.
The circuit configuration described above allows the output transmission power to be fixed to constant power corresponding to the reference voltage of reference voltage generating portion 5, even if the power of the input high-frequency signal is varied in the transmission power control circuit. This enables stable transmission power to be supplied.
However, the conventional transmission power control circuit has a problem in that, when a signal having a frequency band is input, an output high-frequency signal may have an inclination in the frequency property due to the property of the detection efficiency of detection portion 3. Even if the frequency property of the high-frequency signal input into detection portion 3 is flat, the DC voltage output from detection portion 3 may be inclined as shown in FIG. 10A, since the detection efficiency of detection portion 3 has a property such that the efficiency becomes smaller as the frequency becomes higher. Thus, comparator portion 4 recognizes that the power of a high frequency is smaller. As a result, the power of the high-frequency output of transmission-level adjusting portion 1 has an inclined frequency property such that the power is smaller in lower frequency and larger in higher frequency.
It is noted that the detection efficiency of detection portion 3 is different in accordance with a frequency property of a diode, and thus an opposite inclination may be possible depending on the diode used.